Comparisons of Invasive Plants in Southern Africa Originating from Southern Temperate, Northern Temperate and Tropical Regions
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Bothalia 36,2: 201-222 (2006) Comparisons of invasive plants in southern Africa originating from southern temperate, northern temperate and tropical regions L. HENDERSON* Keywords: alien invasive plants, comparisons, southern Africa, temperate and tropical origins ABSTRACT A subset o f invasive alien plant species in southern Africa was analysed in terms o f their history o f introduction, rate of spread, countries/region o f origin, taxonomy, growth forms, cultivated uses, weed status and current distribution in southern Africa, and comparisons made o f those originating from south o f the tropic o f Capricorn, north o f the tropic o f Cancer and from the tropics. The subset o f 233 species, belonging to 58 families, includes all important declared species and some po tentially important species. Almost as many species originate from temperate regions (112) as from the tropics (121). Most southern temperate species came from Australia (28/36), most tropical species from tropical America (92/121) and most northern temperate species from Europe (including the Mediterranean) and Asia (58/76). Transformers account for 33% o f all species. More transformers are of tropical origin (36) than o f northern temperate (24) and southern temperate origin (18). However. 50% of southern temperate species are transformers, compared to 32% o f northern temperate and 29% of tropical species. Southern temperate transformer species are mainly woody trees and shrubs that were established on a grand scale as silvicultural crops, barriers (hedges, windbreaks and screens) and cover/binders. Most aquatics, herbs, climbers and succulent shrubs an. trom the tropics. Ornamentals are the single largest category o f plants from all three regions, the tropics having contributed twice as many species as temperate regions. INTRODUCTION METHODS All terminology relating to invasive plants such as The subset of 233 alien plant species selected for ‘alien’, ‘invasive’, 'naturalized', ‘weed’, ‘environmental this study includes all declared species under CARA, weed' and ‘transformer' are according to Richardson et excluding two hybrids that originated in South Africa al. (2000) unless stated otherwise in the text. More than (Rubus xproteus and Psidium xdurbanensis), and two 1 000 alien plant species are known to be naturalized in eucalypts for which there is little evidence of their inva siveness, Eucalyptus paniculata and E. sideroxylon. The southern Africa (Wells et al. 1986). A high proportion regions of origin were checked against the United States of these species are herbaceous, ruderal and agrestal Department of Agriculture's ARS Germplasm Resources weeds. This paper concentrates on a subset of 233 spe Information Network (GRIN) database, the Missouri cies extracted from the book “Alien weeds and invasive Botanical Garden s MBG: W3TROPICOS database, plants' by Henderson (2001) and which contains all the Mabberley (1997), and other literature sources. major and some of the emerging environmental weeds. Major invaders are those invasive alien species that are Southern temperate species are defined as those species whose region of origin is entirely south of. or well established, and which already have a substantial straddles, the tropic of Capricorn. This region includes impact on natural and semi-natural ecosystems; emerg the South American countries of Uruguay, Argentina. ing invaders currently have less influence but have Chile and southern Brazil. It also includes New Zealand. attributes and potentially suitable habitat that could result Tasmania and Australia (Australian Central Territory, in increased range and consequences in the next few New South Wales, Victoria. South Australia. Western decades (Nel et al. 2004). The list of species selected Australia and southern Queensland). for this study includes virtually all the declared plants Northern temperate species are defined as those spe whose control, propagation and trade are subject to the cies whose region of origin is entirely north of. or strad Conserv ation of Agricultural Resources Act. Act 43 of dles, the tropic of Cancer This region includes Europe, 1983 (CARA), as amended in 2001. North Africa, much of Asia, and North America. Southern Africa has had a long history of plant intro Tropical species are defined as those species whose ductions from various parts of the world (Wells et al. region of origin occurs entirely within the tropics or 1986). This paper aims to compare the plants that have straddles either the tropics of Cancer or Capricorn. This originated from northern temperate, southern temperate region includes tropical America (the northern half of and tropical regions in terms of their history of introduc South America. Central America. Mexico and the West Indies), tropical Africa and Asia (much of India. Thailand tion. rate of spread, countries/region of origin, taxonomy, and Malaysia), Indonesia, and tropical Australia. growth forms, cultivated uses, weed status and current distribution in southern Africa. The earliest dates of occurrence in southern Africa were obtained from specimen records in the Pretoria * Agricultural Research Council: Plant Protection Research Institute, National Herbarium (PRE) and various literature sources. c/o SANBI. Private Bag X I01, 0001 Pretoria The quarter-degree squares occupied and current natural e-mail: hendersonfa sanbi.org ized distributions of the species were obtained from the MS received: 2005-09-08. Southern African Plant Invaders Atlas (SAPIA) data 202 Bothalia 36,2 (2006) base and the Pretoria National Herbarium. The SAPIA The lists of species originating from southern temper database which is housed at the Agricultural Research ate, northern temperate and tropical regions are given in Council (ARC)— Plant Protection Research Institute in Appendices 1, 2 & 3. Pretoria, currently contains 50 000 locality records of more than 500 alien plant species. The database includes records from roadside surveys undertaken from 1979— RESULTS AND DISCUSSION 1993 and from the SAPIA mapping project conducted from January 1994 until December 1998 (Henderson History of introduction of invasive species 1998), with further ad hoc records added to the present. Only 15 species were introduced before 1800 and Weed status was extracted from Henderson (2001). all had their origins in northern temperate and tropical The definitions of the various categories of environmen regions (Figure 1). The earliest species to arrive before tal weeds are after Swarbrick (1991). the colonization of the Cape by the Dutch in 1652, were Environmental weeds Ricinus communis (castor-oil plant) and Achyranthes aspera (burweed), believed to be of tropical African ori Transformers—plants which can dominate or replace gin, and Catharanthus roseus (Madagascar periwinkle). any canopy or subcanopy layer of a natural or seminatu All three species are likely to have had a long association ral ecosystem, thereby altering its structure, integrity and with humans in Africa. Ricinus communis and C. roseus functioning. would have been used for their medicinal value, whereas A. aspera would have been dispersed by domestic live Potential transformers—plants that are already invad stock. There is evidence that R. communis was in the ing natural or seminatural habitats, and have the potential Eastern Cape more than 1 200 years ago (Brink 1988) to dominate any canopy or subcanopy layer but not yet and this begs the question whether it should be regarded having a marked effect. They are either transformers as indigenous and not alien. All three of the aforemen elsewhere in the world or showing signs of this ability in tioned species are widespread in southern Africa but have southern Africa. not become major invaders. Special effect weeds—plants which can significantly The arrival of the Dutch at Cape Town in 1652 marks degrade the value or purpose for which a natural or the start of the introduction of plant species from other seminatural ecosystem is valued without necessarily continents that would eventually become major invaders. dominating it or greatly altering its vegetational structure Seven species arrived between 1652 and 1700. Species or functioning. Examples include weeds which compete of northern temperate origin were: Nasturtium offici with and replace similar native plants, are of high visual nale (watercress), Quercus robur (English oak), Sa/ix impact, poisonous, or have chemical irritants. babylonica (weeping willow), Pinus pinaster (cluster pine) and P. pinea (stone pine). Species of tropical ori Minor weeds—plants that invade and persist in any gin were: Opuntia ftcus-indica (sweet prickly pear) and canopy or subcanopy layer of a natural or seminatural Datura stramonium (common thorn apple). A further five ecosystem but cannot or do not dominate that layer or species arrived before 1800. Arundo donax (giant reed) seriously alter the vegetation structure or its functioning, was the only northern temperate species, whereas spe although the accumulation of several to many species may do so. cies from the tropics were Canna indica (Indian shot), Xanthium spinosum (spiny cocklebur), presumably an Ruderal and agrestal weeds accidental introduction, Opuntia rnonacantha (cochineal prickly pear) and Psidium guajava (guava). Six of the Mostly annual or biennial plants which are primarily species introduced before 1800 (Arundo donax, Opuntia weeds of waste places (ruderals) and cultivated lands ficus-indica, O. rnonacantha, Pinus pinaster, Psidium (agrestals). guajava